The field of the invention is that of motor vehicles. More specifically, the invention relates to the production of axles that are assembled before they are fitted to the vehicles, the assembly of these axles involving fitting rotating parts.
The invention relates most specifically, although not exclusively, to the production of front axles.
Where front axles are concerned, the term “rotating part” is generally used to denote the assembly comprising a stub axle, a hub, a rolling bearing, a disk and a brake caliper.
During the design and production of a rotating part, there is one parameter that is given a special consideration in assessing the quality of the rotating part, and that parameter is the run-out.
The run-out is the parameter that corresponds to the lack of perpendicularity exhibited by the surface of the disk with respect to the axis of rotation of the rotating part (the axis of rotation considered corresponds to that of the rotating part when this part is fitted to the vehicle).
In practice, this run-out is measured on an assembly formed of a hub 1, a rolling bearing 2, a stub axle 3 and a brake disk 4 (as illustrated in FIG. 1). This assembly is rotated and a sensor P measures the run-out 5 mm away from the exterior perimeter of the disk.
It will of course be appreciated that the greater the run-out, the greater the lack of perpendicularity. This results in premature wear of the thickness of one of the tracks of the disk, in the region where the disk run-out is at its greatest.
This localized wear causes a variation in the thickness of the track on the disk and this ultimately causes the brake pads to start to vibrate under braking. This vibration can then be felt by the driver, through the steering wheel.
At the present time, the manufacturers of mass-produced motor vehicles will tolerate run-out of up to 60 μm on the rotating part, and this corresponds to an error of the order of 25 μm on the disk/hub bearing surface.
However, new requirements are gradually emerging, even for private motor cars, and are leading to an appreciable reduction in the run-out so that the error in perpendicularity measured on the disk/hub bearing surface is 10 μm at most, so as to obtain a disk run-out of a maximum of 40 μm on the assembled rotating part.
Such a reduction (by 15 μm) is considerable given the number of parts that are assembled in order to form the assembly on which the run-out is measured (which, as already mentioned, corresponds to the lack of perpendicularity at the disk/hub bearing surface). This is because each of the parts is produced with its own manufacturing tolerances, which makes it very difficult to control the run-out once all these dimensions have been combined together.
A solution for reducing the run-out in a rotating part has been proposed in the prior art.
As already mentioned, traditionally the rotating part comprised a hub onto which a rolling bearing is fitted. This rolling bearing comprises an outer cage (on which the stub axle is mounted) and an inner cage (within which part of the hub is fitted).
The prior-art solution for reducing run-out was to reduce the number of parts in the combination of dimensions of the rotating part. To do that, the inner cage of the rolling bearing was omitted, the hub taking its place. In other words, the balls of the rolling bearing are mounted directly between the hub and an outer cage.
These rolling bearings of a new type, known as “3rd-generation rolling bearings” are able to meet the new requirements imposed by manufacturers in terms of rotating part run-out.
However, these rolling bearings prove to be very expensive and therefore have a considerable impact on the cost of the assembled axle with its rotating parts.